In the manufacture of drawn and ironed containers, it is standard practice to cut a circular disk from a piece of stock material and form a cup in a cupping machine. The cup is then transferred to a bodymaker wherein the side wall thickness of the cup is reduced and the height is increased. At the end of the stroke of the bodymaker, it is customary to provide an inwardly domed configuration to the end wall of the container to provide increased pressure resistance internally of the container.
Numerous inspections are necessary for assuring that the various components of the drawn and ironed container meet the rigid specifications that have been developed by purchasers of such containers which ultimately use them for packaging products, particularly beer and beverages.
One of the critical parameters of a drawn and ironed container is the depth of the integral dome in relation to the chime of the container. If the dome depth is less than the critical limits, the structural rigidity of the container does not meet minimum requirements. A shallow dome many times cannot be visually observed and can be filled and sealed without detection. Such container will mostly likely "buckle" while being stored in a warehouse where it is subjected to significant temperature changes resulting in internal pressure changes. In other instances, the container might "buckle" when the contents is pasteurized.
Numerous proposals have been made for measuring the various critical dimensional requirements of a drawn and ironed container, but most checking and testing units are rather complicated in nature including electronic equipment and in most instances require that the container be taken out of the processing line in a can manufacturing plant. Such an arrangement adds to the overall cost of the container and reduces the efficiency of the operation.
Of course, it will be appreciated that, when containers are being produced on a mass production at speeds of 200 containers per minute, it is essential that any defect is detected immediately to reduce the number of containers that must be discarded.